The present invention relates to an emission coil for emitting electromagnetic waves of a predetermined frequency to an RF coil mounted on a subject being carried in a static magnetic field and an MRI apparatus for imaging a magnetic resonance image by receiving a magnetic resonance signal emitted from the subject, and more specifically to a method of coil element selection of a magnetic resonance imaging apparatus which allows selecting the optimum coil element suitable for MR measurement from within a plurality of coils embedded in a phased array coil, in accordance with the lesion to be imaged.
There has been known a magnetic resonance imaging apparatus (MRI apparatus herein below) for imaging the internal structure of a subject to be imaged by means of magnetic resonance phenomenon hitherto. Since the magnetic resonance phenomenon is harmless to a living body, the MRI apparatus is useful in medical field and used in the detailed examination of whole body system and in the diagnosis of brain tumor.
The magnetic resonance phenomenon is a phenomenon that, in an object having a uniform static field applied, the spin axis of nuclei of atoms constituting an object aligns to absorb and release the electromagnetic waves of a frequency in proportion to the intensity of static field. The MRI apparatus uses the magnetic resonance phenomenon with respect to a specific specie of nuclei (primarily to hydrogen atoms) to image a given section of the object being imaged at a given thickness.
For the RF coil in the MRI apparatus, hitherto, a phased array coil is used, which may have a larger imaging area without the need of elongating the imaging time for the MRI diagnosis. In this case, it is needed to select the coil element that is in position suitable for the object being imaged (because of the highest sensitivity in the imaging area) from within a plurality of coil elements constituting the phased array coil, in order to perform the imaging with the coil element.
In practice, a manufacturer of medical apparatus fixes in advance the combination of which coil elements to be used from within the coil elements constituting the phased array coil, and notifies the imaging technician of the coil sensitivity of each combination prior to the practical use.
More specifically, the imaging technician selects a combination of coils which is best suited to a clinical use, on the basis of the information and his/her experience, or changes the combination of coil elements if there is discrepancy with the imaging area, or changes and adjusts the relative position of the subject to the coil in order to achieve the MRI imaging.
As the Prior Art of this field, for example that disclosed in the patent reference 1, there is disclosed a technique of magnetic resonance imaging method by selecting an optimum combination of RF receiver coils in accordance with the imaging section, combining the signals received with the RF receiver coils in response to the combination selected, and eliminating unnecessary part of image with a matrix computation by using thus bound signals. Also in the patent reference 2, there is disclosed a technique of determining a coil with the maximum available sensitivity to be selected and a coil with the minimum available sensitivity to be omitted, based on the determination of corresponding index gauge collected during the preliminary scan with respect to a plurality of coils selected in advance.
Patent reference 1: JP-A-2002-248089
Patent reference 2: JP-A-2002-355233
The MRI apparatus of the prior art as have been described above, however, may have the problems as cited below. More specifically, the MRI apparatus of the prior art, as stated above, requires the adjustment of position of the phased array coil to be mounted on the subject based on the experience of the imaging technician in order to achieve the MRI imaging.
However, as the number of channels constituting the phased array coil increases, the degree of freedom that which elements (constituting coils) of a phased array coil to be selected also increases. This poses a problem of difficulty of selecting the optimum coil element most suitable for the practical MRI measurement.
In other words, the sensitivity direction of coil elements is fixed to a predefined direction in accordance with the form of each respective coil. In order to adjust the sensitivity direction of coil element therefore a work is needed for changing the coil element disposition, leading to a problem that the burden of the amount of work of the imaging technician and physician increases, as well as to another troublesome problem that after having an MRI imaging session, the MRI imaging should be run again if the mounting position of phased array coil is not appropriate.